Some communication devices perform transmission using an adaptive array antenna system that adaptively controls directivity of an array antenna composed of multiple antennas. Such an adaptive array antenna system may comprise beam-forming/beam-steering and null-steering. In the beam-forming, a transmission directivity of an array antenna is controlled so that a beam is faced in a desired direction (e.g., a direction in which a communication partner device is located), and null control is intentionally not performed. Alternatively, in null-steering, the transmission directivity of the array antenna is controlled so that the null faces in a direction in which interference should not be imparted during transmission (e.g., a direction in which a communication device other than the communication partner device is located; hereinafter referred to as a “direction of suppressed interference”), and beam control is intentionally not performed.
In null-steering, because a null where transmission gain decreases is formed in the transmission directivity of the array antenna due to the effects of the null, a transmission gain in the desired direction tends to be small compared to a transmission gain for beam steering. Similarly, when simultaneously performing null-steering and beam-forming, transmission gain in the desired direction tends to be small compared to when only beam steering is performed. Therefore, the transmission performance of the communication device may be degraded.